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Biochar adsorbs contaminants from wastewater. However, biochar's modification, its application in nutrient removal from wastewater, and its mechanism of nutrient removal need to be explored. In this study, nitrate and phosphate were recovered from secondary treated domestic wastewater using microalgal biochar (Spirulina sp.). The biochar was obtained by microwave pyrolysis of algae at 400 W power for 10 min. The biochar was modified using Mg(OH)2 under different conditions. Three pre‐and post‐treated biochars with varied Mg(OH)2 percentages (2%, 4%, and 6% by weight) were produced. All the modifications in biochar improved its specific surface area. Highest specific surface area of the modified biochar reached 33 m2/g compared to 12 m2/g of the unmodified biochar. SEM analysis showed improvement in the porosity of the modified biochars. Modification also introduced ‒OH, ‒C=C‒ and ‒C‒O‒C‒ functional groups on the biochar surface. All these modifications improved the nutrient adsorption capability of the biochar. Adsorption studies indicated that 4% post‐treated biochar (Po4) was the best. The adsorption data fit the pseudo second order kinetic model with R2 being 0.99 and 0.96 for phosphate and nitrate respectively. Among different isotherm models, the Freundlich isotherm model showed the best fit for phosphate and nitrate, with their respective R2 being 0.96 and 0.97, indicating multi‐layer physical‐chemical adsorption, including the intra‐particle diffusion. The equilibrium time for adsorption for nitrate and phosphate was 7 and 6 h, respectively. The optimum pH was 5 and 6 for nitrate and phosphate, respectively, and the optimum dosage was 2 g/L. The maximum adsorption capacity for nitrate and phosphate was 7.9 and 8.9 mg/g, respectively. Po4 showed potential as a fertilizer as it had 5.84 and 3.02 g of extractable phosphate and nitrate per kg of biochar.